Optimizing the stimulus current in a surface based stimulation device

ABSTRACT

A method and associated stimulation device for ensuring firing of an action potential in an intended physiological target activated by a stimulus pulse generated by an electrode of a non-invasive surface based stimulation device irrespective of skin-to-electrode impedance by: (i) increasing internal impedance of the stimulation device so as to widen a Chronaxie time period thereby ensuring firing of the action potential of the intended physiological target irrespective of the skin-to-electrode impedance; and/or (ii) generating a stimulation waveform that optimizes a non-zero average current (e.g., non-zero slope of the envelope of the stimulation waveform) during preferably substantially the entire current decay of the stimulus pulse.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to an improved surface basedstimulation device that optimizes the stimulus current delivered whileminimizing its duration resulting in higher efficiency and minimum powerconsumption.

2. Description of Related Art

A nerve cell can be excited externally by increasing the electricalcharge within the nerve, thus increasing the membrane potential insidethe nerve with respect to the surrounding extracellular fluid. U.S.patent application Ser. No. 11/146,522, filed on Jun. 7, 2005 andassigned to the same assignee as the present application, discloses anexternal transdermal nerve stimulation patch. This is but one example ofa surface based stimulation device. The fundamental feature of thenervous system, i.e., its ability to generate and conduct electricalimpulses, can take the form of action potentials (AP), which are definedas a single electrical impulse passing down an axon or fiber. Thisaction potential (sometimes also referred to as a nerve impulse orspike) is an “all or nothing” phenomenon. Rheobase is the minimalelectrical current of infinite duration (practically, a few hundredmilliseconds) that results in an action potential. In addition, to aminimum intensity there is also a minimum amount of time necessary toexcite the nerve. The minimum time referred to as Chronaxie is aduration of time that produces a response when the nerve is stimulatedat twice the Rheobase strength. If either the stimulation time orstimulation intensity of the stimulation signal is not sufficient, thenerve will not fire an action potential. An exemplary strength-durationcurve for excitable tissue shown in FIG. 1 denotes the Rheobase byreference element “1” and has a value of 0.25 V while the Chronaxie isrepresented by reference element “3” and has a value of approximately0.23 ms.

When an external electrical stimulus is applied transcutaneously thecomplex impedance of the skin can alter the stimulus current. Forexample, if the capacitance of the skin increases, the amplitude of thestimulation signal may be sufficient to excite a nerve, however, anadequate amount of time may be lacking to fire the action potential.This is known as current decay. When a DC current is applied to thesurface of the skin, a decay in current is observed until the netcurrent is zero. Typically, this takes approximately 600 microseconds.

It is therefore desirable to develop an improved method and system thatadjusts for changing impedance of the skin or decay in current.

SUMMARY OF THE INVENTION

An aspect of the present invention is directed to a system and methodfor widening or increasing the current decay to insure that the actionpotential will fire without increasing the amplitude or strength of thestimulation signal.

Another aspect of the present invention relates to a system and methodfor increasing the time allotted to the stimulus signal to fire theaction potential and therefore prolong the decay in the current byadding an in-line series impedance on the electrode interface side.

Still another aspect of the present invention is directed to a systemand method for increasing the time allotted to the stimulus signal tofire the action potential by employing a stimulus waveform envelopehaving a non-zero slope or rate of change at least during substantiallythe duration of current decay so that the average current is non-zero.

The present invention is directed to a method and associated stimulationdevice for ensuring firing of an action potential in an intendedphysiological target activated by a stimulus pulse generated by anelectrode of a non-invasive surface based stimulation deviceirrespective of skin-to-electrode impedance by: (i) increasing internalimpedance of the stimulation device so as to widen a Chronaxie timeperiod thereby ensuring firing of the action potential of the intendedphysiological target irrespective of the skin-to-electrode impedance;and/or (ii) generating a stimulation waveform that optimizes a non-zeroaverage current (e.g., non-zero slope of the envelope of the stimulationwaveform) during preferably substantially the entire current decay ofthe stimulus pulse.

One particular aspect of the present invention is directed to a methodfor ensuring firing of an action potential in an intended physiologicaltarget activated by a stimulus pulse generated by an electrode of anon-invasive surface based stimulation device irrespective ofskin-to-electrode impedance. This is accomplished by increasing aninternal impedance of the stimulation device. The stimulus pulse isgenerated by the electrode. Accordingly, the increased internalimpedance widens a Chronaxie time period thereby ensuring firing of theaction potential of the intended physiological target irrespective ofthe skin-to-electrode impedance.

Another specific aspect of the present invention is directed to astimulation device for realizing the method described in the precedingparagraph. The stimulation device includes a waveform generator forproducing a stimulation waveform. An electrode is electrically connectedto receive the stimulation waveform and produce a stimulus pulse. Aresistor in series with the electrode is provided for increasinginternal impedance of the stimulation device, wherein the increasedinternal impedance widens a Chronaxie time period so as to ensure firingof the action potential of the intended physiological targetirrespective of the skin-to-electrode impedance.

Still another particular aspect of the present invention is directed toa method for ensuring firing of an action potential in an intendedphysiological target activated by a stimulus pulse generated by anelectrode of a non-invasive surface based stimulation deviceirrespective of skin-to-electrode impedance. A stimulation waveform isgenerated that optimizes a non-zero average current (e.g., non-zeroslope of the envelope of the stimulation waveform) during current decayof the stimulus pulse. The stimulus pulse is produced using theelectrode that receives as input the stimulation waveform.

Yet another aspect of the present invention is directed to a stimulationdevice for realizing the method described in the preceding paragraph.The stimulation device includes a waveform generator for producing astimulation waveform that optimizes a non-zero average current duringcurrent decay of the stimulus pulse. An electrode receives as input thestimulation waveform and generates the stimulus pulse.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing and other features of the present invention will be morereadily apparent from the following detailed description and drawings ofillustrative embodiments of the invention wherein like reference numbersrefer to similar elements throughout the several views and in which:

FIG. 1 is an exemplary strength—duration curve for stimulation oftissue;

FIG. 2 is an exemplary schematic stimulation circuit simulating the skinto electrode interface and observing the differentiation current;

FIG. 3 shows exemplary voltage—time simulation results for the simulatedstimulation circuit in FIG. 2 wherein the stimulation waveform is asquare wave at a 20 Hz stimulation frequency;

FIG. 4 shows exemplary voltage—time simulation results for the simulatedstimulation circuit in FIG. 2 wherein the stimulation waveform is asquare wave at a 210 KHz stimulation frequency;

FIG. 5 a is an exemplary schematic stimulation circuit compensating forthe skin-electrode impedance by adding an in-line series resistor;

FIG. 5 b shows exemplary voltage—time simulation results for thesimulated stimulation circuit in FIG. 5a wherein the stimulationwaveform is a square wave at 20 Hz stimulation frequency;

FIG. 5 c shows exemplary voltage—time simulation results for thesimulated stimulation circuit in FIG. 5 a wherein the stimulationwaveform is a square wave at 210 KHz stimulation frequency;

FIG. 5 d is an exemplary schematic stimulation circuit compensating forthe skin-electrode impedance by adding a parallel resistance;

FIG. 6 is an exemplary schematic diagram of a transcutaneous externalsstimulation device for adjusting the envelope of the stimulation signalbased on feedback of detected skin impedance;

FIG. 7 is an exemplary voltage—time simulation result for the simulatedstimulation circuit in FIG. 2 wherein the stimulation waveform is apredetermined ramped waveform; and

FIG. 8 is an exemplary voltage—time simulation result for the simulatedstimulation circuit in FIG. 2 wherein the stimulation waveform is apredetermined exponential waveform.

DETAILED DESCRIPTION OF THE INVENTION

The present inventive system and method is an improvement for anon-invasive surface based stimulation device wherein an electrode ispositioned proximate or in direct contact with the surface of the skinto externally stimulate a physiologic target, e.g., nerve or tissue tobe treated. When an electrical stimulus is applied through the skinusing external electrodes the complex impedance of the skin can alterthe stimulus current. The impedance of the skin is changing constantlyand is dependent on numerous factors. For instance, skin impedancegradually rises with advancing age, and females tend to show highervalues then males. A significant correlation has been documented betweenthe percentage decrease in skin impedance in response to strain on thejoint such as during standing, bending, squatting, walking on a flatfloor, and ascending/descending stairs. In addition, skin temperature,presence of hair and skin type also contribute to changes in impedance.If the impedance of the skin alters the stimulus current so that theamount of time needed to excite the nerve (Chronaxie) is not reached,the nerve will not be stimulated (i.e., the action potential will not befired) and the stimulation device will therefore not be effective intreating the condition intended to be treated.

Therefore, a minimum stimulus duration is required to activate or firethe action potential. However, when the stimulus is provided viatranscutaneous electrodes, the complex impedance of the skin can alterthe stimulus current. The effect of the impedance at the electrode toskin interface may be approximated as a differentiation. That is, theskin impedance is differentiating the stimulus waveform and thereforethe stimulus current. A resistance-capacitance circuit (RC circuit) maybe utilized to simulate the bulk resistance and capacitance of theskin-electrode impedance. FIG. 2 is an exemplary schematic circuit forsimulating the skin-electrode impedance using an RC circuit. A waveformgenerator 105 represents the stimulus signal. The stimulus signalproduced by waveform generator 105 passes through a series RC circuit(R1 and C1) that represents the bulk resistance and capacitance of theskin to electrode interface. The differentiated current is observedusing an oscilloscope 110.

In an illustrative example, the resistance and capacitance values areset to R1=619 Ohms and C1=470 nF, respectively. A voltage—time trace ofthe simulation results in FIG. 2 is shown in FIG. 3 wherein a squarewave stimulus signal is produced by waveform generator 105 at a stimulusfrequency of 20 Hz. The top waveform represents the 20 Hz square wavestimulus produced by waveform generator 105 while the lower waveformdepicts the differentiated current as observed by oscilloscope 110. Thepulse width (duration) of the stimulus signal is approximately 25 ms,whereas the pulse width (duration) of the differentiated current is onlyapproximately 2 ms at both the rising and trailing edges of thestimulus. For the remaining 21 ms of the stimulus pulse, no current isrecorded. By way of another example, the square wave stimulus signalproduced by waveform generator 105 is set to 210 KHz and the stimulationtrace results are shown in FIG. 4. A positive net average current can beseen from time t0 until approximately 600 microseconds. The averagecurrent is approximately 0 mA after the initial 600 microseconds.Stimulation beyond 600 microseconds has a zero average current. In theillustrative examples shown in FIGS. 3 and 4, the duration of thedifferentiated current (Chronaxie) may not be sufficient to activate theintended or target excitable tissue. Thus, it is desirable to increasethe duration of the stimulation current in order to fire the actionpotential of the intended or target excitable tissue.

One possible solution to overcome the loss of stimulus duration due tothe skin-electrode impedance is increasing the strength of the stimulussignal above the required Rheobase. Referring once again to thestrength—duration waveform in FIG. 1, the stimulus strength required toactivate excitable tissue occurring at durations below the Chronaxie aresignificantly higher values than those occurring above the Chronaxie.Such relatively high stimulus signal strengths may undesirably result inpossible activation of a non-targeted tissue. In addition, the increasedstimulation signal strength will consume more power which may bedisadvantageous especially when the external stimulating device has alimited power supply.

The present inventive system and method compensates for changing orvarying skin to electrode impedance as well as the decay in current bywidening the decay, i.e., increasing the time, so that the nerve fireswithout having to increase the power of the device. By increasing thetime to allow the stimulus to fire the nerve the decay in current isprolonged. The decay can be widened or increased, in other words moretime is added, by either including additional series impedance and/or byincreasing the amplitude of the stimulation waveform during the decay.

It has been recognized that by controlling the impedance of theelectrode to skin interface the duration of the stimulus current can beincreased or decreased slightly above the Chronaxie of the intendedexcitable tissue or slightly below the Chronaxie of the unintendedexcitable tissue. That is, in-line series resistance (R2) can be addedon the electrode side of the interface to shift the stimulus pulseduration above the Chronaxie of the intended physiological target (e.g.,excitable tissue), as shown in FIG. 5 a. For example, a resistance ofseveral hundred ohms may be added to slightly increase the impedance sothat the stimulus pulse duration is shifted above the Chronaxie.Conversely, the series resistance (R2) can be removed or insteadparallel resistance added (R3)(as represented in FIG. 5 d) from theelectrode side of the interface to shift the stimulus pulse durationbelow the Chronaxie of the unintended excitable tissue. In this way, thestrength of the stimulus signal may be compensated so that the target orintended tissue is stimulated even if the impedance of the skin changes.It is also contemplated and within the intended scope of the presentinvention to control the resistance on a real time basis by detectingthe impedance of the skin and adjusting a variable in-line seriesresistor based on the detected impedance. Preferably the impedance ofthe skin in sampled or monitored in real time and from this measuredvalue is derived the resistance and capacitance (current decay). Theimpedance of the skin may be measured using conventional schemes such asapplying a current source (constant or variable) and measuring the skinimpedance using a sensor or electrode place on the surface of the skin.The variable resistance and thus stimulus pulse duration is preferablyvaried in direct proportion to that of the detected skin-electrodeimpedance. That is, as the detected skin-electrode impedance increasesthe variable resistance is increased to lengthen the duration of thestimulus pulse to allow a longer period of time for stimulation, whereasas the impedance decreases the variable resistance is decreased therebyshortening the duration of the stimulus pulse.

FIG. 5 b is an exemplary stimulation trace in which a 1 kΩ in-lineseries resistor R2 was added and the simulation was re-run at 20 Hz.Once again the upper waveform represents the stimulation signal whereasthe lower waveform represents the differentiated current. The currentpulse of the waveform shown in FIG. 5 b is now 3 ms in durationrepresenting an increase of 1 ms from the baseline case with no seriesresistance in FIG. 2 in which the current pulse waveform was 2 ms induration.

FIG. 5 c shows the trace of the stimulus current waveform at a 210 KHzwith the same in-line series resistance of 1 kΩ. A current pulseduration greater than zero can be seen for 2.5 ms representing a 1.9 msincrease above the baseline of 600 microseconds (0.6 ms), whichpreviously represented an average current of approximately 0 mA in FIG.4. These exemplary stimulus current waveform traces verify that theaddition of an in-line series resistor increases the duration of thestimulus pulse to above the threshold for excitation of nerves (in thisexample 600 microseconds) so that a nerve will be stimulated even if theimpedance of the skin changes. To minimize power consumption, preferablythe minimum amount of in-line series resistance to achieve excitation ofa nerve is used. However, if power consumption is not a factor orconsideration of a particular circuit design, the in-line resistancevalue need not be minimized.

In those circumstances in which power consumption is a considerationsuch as with a limited power source (e.g., battery) increased impedancevia addition of any in-line series resistance will disadvantageouslydissipate or consume some power thereby shortening the lifespan of thepower source. An alternative method for widening the decay whileminimizing the power consumed is to use a waveform for the stimulussignal whose envelope has a non-zero slope or rate of change at leastduring the current decay. Referring once again to FIG. 3 in which asquare wave stimulus signal having a zero slope envelope was used adifferentiation current was present for only a very small pulse width(duration) of several milliseconds while for the remainder of thestimulus pulse, no current was present. The slope or rate of change ofthe stimulus pulse envelope is directly proportional to the duration ofnon-zero current. It is therefore desirable to employ a stimuluswaveform whose envelope has a non-zero slope or rate of change at leastduring current decay of the waveform to optimize the duration of thenon-zero current. Preferably, a stimulus waveform is used whose envelopehas a non-zero slope or rate of change over substantially the durationof current decay of the waveform, and most preferably over substantiallythe entire stimulus pulse width or interval.

Instead of a square wave stimulus having zero slope, a few exemplarypredetermined waveforms whose envelopes have a non-zero slope or ratechange during the current decay were utilized and the results observed.In a first example, a ramped stimulus was employed using the simulationsetup in FIG. 2 at a frequency of 20 Hz, the results of which aredepicted in FIG. 7. The ramped stimulus is the top trace while thedifferentiated current trace is the bottom trace. The pulse width(duration) of the ramped stimulus is approximately 30 ms. The durationof non-zero current can be seen for the duration of the stimulus and isproportional to the slope or rate of change of the applied stimulus.

Yet another example employing an exponential stimulus waveform using thesimulation circuitry in FIG. 2 was also run at a frequency of 20 Hz. InFIG. 8, the exponential stimulus is the top trace, while thedifferentiated current trace is the bottom trace. The pulse width(duration) stimulus is approximately 25 ms. Once again, the duration ofnon-zero current can be seen for the duration of the stimulus and isproportional to the slope or rate of change of the applied stimulus.

The slope or rate of change of the applied stimulus envelope ispreferably selected to match and thus compensate for the skin impedance.Since the skin impedance may vary over time due to any number of factorspreviously mentioned it would be desirable to adjust the slope or rateof change of the applied stimulus envelope on a real time basis to matchthe detected capacitance rather than employ a predefined stimuluswaveform. Therefore, instead of applying as the stimulation signal apredetermined stimulation envelope whose slope at least duringsubstantially the duration of current decay is non-zero, the slope ofthe stimulus envelope may be adjusted on a real time basis at leastduring substantially the duration of current decay to compensate for themonitored skin to electrode impedance. As the detected skin-electrodeimpedance increases the slope of the envelope is increased to widen thedecay thereby allowing a longer duration for stimulation, whereas as thedetected skin-electrode impedance decreases the slope of the envelope isdecrease thereby shortening the decay and duration for stimulation.

An exemplary schematic diagram of a surface based stimulation device 600in accordance with the present invention that adjusts the envelope ofthe stimulation signal waveform on a real time basis as a function ofdetected skin impedance feedback is shown in FIG. 6. Preferably theimpedance of the skin in sampled or monitored in real time and from thismeasured value is derived the resistance and capacitance (currentdecay). The impedance of the skin may be measured using conventionalschemes such as applying a current source (constant or variable) andmeasuring the skin impedance using a sensor 620 or electrode placed onthe surface of the skin. Depending on the amount or intensity of themonitored decay, the surface based stimulation device includes aprocessor or controller 625 powered by a power source 615 and programmedto adjust during the current decay the shape of the waveform (e.g.,slope of the pulse envelope) produced by the waveform generator 605 inreal time to compensate for this measured current decay. The adjustedcurrent stimulus produced by the waveform generator 605 is received asinput to the external electrode 610 to stimulate the target (e.g.,nerve).

Therefore, the stimulus signal may be compensated for skin electrodeimpedance by adding/eliminating an in-line series resistance or bychanging the shape of the envelope of the stimulus waveform so that itsaverage current is non-zero during current decay. It is alsocontemplated and within the intended scope of the present invention forthese two options to be utilized simultaneously to compensate for anyvariation in the stimulation current due to impedance of the skin.

Thus, while there have been shown, described, and pointed outfundamental novel features of the invention as applied to a preferredembodiment thereof, it will be understood that various omissions,substitutions, and changes in the form and details of the devicesillustrated, and in their operation, may be made by those skilled in theart without departing from the spirit and scope of the invention. Forexample, it is expressly intended that all combinations of thoseelements and/or steps that perform substantially the same function, insubstantially the same way, to achieve the same results be within thescope of the invention. Substitutions of elements from one describedembodiment to another are also fully intended and contemplated. It isalso to be understood that the drawings are not necessarily drawn toscale, but that they are merely conceptual in nature. It is theintention, therefore, to be limited only as indicated by the scope ofthe claims appended hereto.

Every issued patent, pending patent application, publication, journalarticle, book or any other reference cited herein is each incorporatedby reference in their entirety.

1. A method for ensuring firing of an action potential in an intendedphysiological target activated by a stimulus pulse generated by anelectrode of a non-invasive surface based stimulation deviceirrespective of skin-to-electrode impedance, comprising the step of:increasing internal impedance of the stimulation device; and generatingthe stimulus pulse using the electrode; wherein the increased internalimpedance widens a Chronaxie time period so as to ensure firing of theaction potential of the intended physiological target irrespective ofthe skin-to-electrode impedance.
 2. The method in accordance with claim1, wherein the increasing step comprises the step of shifting a durationof the stimulus pulse above the Chronaxie of the intended physiologicaltarget required for firing of the action potential.
 3. The method inaccordance with claim 1, wherein the increasing step comprises adding aresistor in series with the electrode.
 4. The method in accordance withclaim 3, further comprising the step of in the presence of a reductionin the skin-to-electrode impedance, decreasing the internal impedance byremoving the series resistor.
 5. The method in accordance with claim 3,in the presence of a reduction in the skin-to-electrode impedance,further comprising the step of decreasing the internal impedance byadding a parallel resistor across the electrode.
 6. The method inaccordance with claim 3, wherein the resistor is several hundred ohms.7. A non-invasive surface based stimulation device that ensures firingof an action potential in an intended physiological target irrespectiveof skin-to-electrode impedance, comprising: a waveform generator forproducing a stimulation waveform; an electrode electrically connected toreceive the stimulation waveform and produce a stimulus pulse; and aresistor in series with the electrode for increasing internal impedanceof the stimulation device, wherein the increased internal impedancewidens a Chronaxie time period so as to ensure firing of the actionpotential of the intended physiological target irrespective of theskin-to-electrode impedance.
 8. The stimulation device in accordancewith claim 7, wherein the resistor shifts a duration of the stimuluspulse above the Chronaxie of the intended physiological target requiredfor firing of the action potential.
 9. The stimulation device inaccordance with claim 7, further comprising: a resistor in parallel withthe electrode for decreasing the internal impedance of the stimulationdevice, wherein the decreased internal impedance shortens the Chronaxietime period in response to a decrease in the skin-to-electrodeimpedance.
 10. The stimulation device in accordance with claim 7,wherein the resistor is several hundred ohms.
 11. A method for ensuringfiring of an action potential in an intended physiological targetactivated by a stimulus pulse generated by an electrode of anon-invasive surface based stimulation device irrespective ofskin-to-electrode impedance, comprising the steps of: generating astimulation waveform that optimizes a non-zero average current duringcurrent decay of the stimulus pulse; and producing the stimulus pulseusing the electrode that receives as input the stimulation waveform. 12.The method in accordance with claim 11, wherein the generating stepcomprises selecting the stimulation waveform forming an envelope havinga non-zero slope during at least a portion of the current decay.
 13. Themethod in accordance with claim 12, wherein the non-zero slope of theenvelope of the stimulation waveform extends over approximately 50% ofthe current decay.
 14. The method in accordance with claim 13, whereinthe non-zero slope of the envelope of the stimulation waveform extendsover substantially the entire current decay.
 15. The method inaccordance with claim 11, wherein the stimulation waveform is apredefined waveform.
 16. The method in accordance with claim 15, whereinthe predefined waveform is one of a ramp or an exponential waveform. 17.The method in accordance with claim 11, wherein before the generatingstep, further comprising the steps of: detecting in real time theskin-to-electrode impedance; and determining the current decay based onthe detected skin-to-electrode impedance.
 18. The method in accordancewith claim 17, wherein the producing step comprises the step ofadjusting in real time during the current decay the slope of thestimulation waveform so as to substantially match the detectedskin-to-electrode impedance.
 19. The method in accordance with claim 18,wherein the adjusting step comprises increasing the slope of theenvelope of the stimulation waveform as the detected skin-to-electrodeimpedance increases, while decreasing the slope of the envelope of thestimulation waveform as the detected skin-to-electrode impedancedecreases.
 20. A non-invasive surface based stimulation device ensuringfiring of an action potential in an intended physiological targetactivated by a stimulus pulse generated by an electrode irrespective ofskin-to-electrode impedance, comprising: a waveform generator forproducing a stimulation waveform that optimizes a non-zero averagecurrent during current decay of the stimulus pulse; and an electrodereceiving as input the stimulation waveform and generating the stimuluspulse.
 21. The stimulation device in accordance with claim 20, whereinthe stimulation waveform forms an envelope having a non-zero slopeduring at least a portion of the current decay.
 22. The stimulationdevice in accordance with claim 21, wherein the non-zero slope of theenvelope of the stimulation waveform extends over approximately 50% ofthe current decay.
 23. The stimulation device in accordance with claim22, wherein the non-zero slope of the envelope of the stimulationwaveform extends over substantially the entire current decay.
 24. Thestimulation device in accordance with claim 20, wherein the stimulationwaveform is a predefined waveform.
 25. The stimulation device inaccordance with claim 24, wherein the predefined waveform is one of aramp or an exponential waveform.
 26. The stimulation device inaccordance with claim 20, further comprising: circuitry for detecting inreal time the skin-to-electrode impedance; and a processor fordetermining the current decay based on the detected skin-to-electrodeimpedance.
 27. The stimulation device in accordance with claim 26,wherein the processor adjusts in real time during the current decay theslope of the stimulation waveform so as to substantially match thedetected skin-to-electrode impedance.
 28. The stimulation device inaccordance with claim 27, wherein the processor increases the slope ofthe envelope of the stimulation waveform as the detectedskin-to-electrode impedance increases, and decreases the slope of theenvelope of the stimulation waveform as the detected skin-to-electrodeimpedance decreases.
 29. The method in accordance with claim 1, whereinthe step of generating the stimulus pulse comprises the steps of:generating a stimulation waveform that optimizes a non-zero averagecurrent during current decay of the stimulus pulse; and producing thestimulus pulse using the electrode that receives as input thestimulation waveform.
 30. The method in accordance with claim 29,wherein the step of generating the stimulation waveform comprisesselecting the stimulation waveform forming an envelope having a non-zeroslope during at least a portion of the current decay.
 31. The method inaccordance with claim 30, wherein the non-zero slope of the envelope ofthe stimulation waveform extends over approximately 50% of the currentdecay.
 32. The method in accordance with claim 31, wherein the non-zeroslope of the envelope of the stimulation waveform extends oversubstantially the entire current decay.
 33. The method in accordancewith claim 29, wherein the stimulation waveform is a predefinedwaveform.
 34. The method in accordance with claim 33, wherein thepredefined waveform is one of a ramp or an exponential waveform.
 35. Themethod in accordance with claim 29, wherein before the step ofgenerating the stimulation waveform, further comprising the steps of:detecting in real time the skin-to-electrode impedance; and determiningthe current decay based on the detected skin-to-electrode impedance. 36.The method in accordance with claim 29, wherein the producing stepcomprises the step of adjusting in real time during the current decaythe slope of the stimulation waveform so as to substantially match thedetected skin-to-electrode impedance.
 37. The method in accordance withclaim 36, wherein the adjusting step comprises increasing the slope ofthe envelope of the stimulation waveform as the detectedskin-to-electrode impedance increases, while decreasing the slope of theenvelope of the stimulation waveform as the detected skin-to-electrodeimpedance decreases.
 38. The stimulation device in accordance with claim7, further comprising: a waveform generator for producing a stimulationwaveform that optimizes a non-zero average current during current decayof the stimulus pulse; wherein the electrode receives as input thestimulation waveform and generates the stimulus pulse.
 39. Thestimulation device in accordance with claim 38, wherein the stimulationwaveform forms an envelope having a non-zero slope during at least aportion of the current decay.
 40. The stimulation device in accordancewith claim 39, wherein the non-zero slope of the envelope of thestimulation waveform extends over approximately 50% of the currentdecay.
 41. The stimulation device in accordance with claim 40, whereinthe non-zero slope of the envelope of the stimulation waveform extendsover substantially the entire current decay.
 42. The stimulation devicein accordance with claim 38, wherein the stimulation waveform is apredefined waveform.
 43. The stimulation device in accordance with claim42, wherein the predefined waveform is one of a ramp or an exponentialwaveform.
 44. The stimulation device in accordance with claim 38,further comprising: circuitry for detecting in real time theskin-to-electrode impedance; and a processor for determining the currentdecay based on the detected skin-to-electrode impedance.
 45. Thestimulation device in accordance with claim 44, wherein the processoradjusts in real time during the current decay the slope of thestimulation waveform so as to substantially match the detectedskin-to-electrode impedance.
 46. The stimulation device in accordancewith claim 45, wherein the processor increases the slope of the envelopeof the stimulation waveform as the detected skin-to-electrode impedanceincreases, and decreases the slope of the envelope of the stimulationwaveform as the detected skin-to-electrode impedance decreases.